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You may know Harlan Howe from his twelve years as publisher and editor of Microwave Journal ®, or from his 34 years as a Microwave design engineer and engineering manager, or from his service as an IEEE fellow and past president of MTT-S.

Now, although semi-retired, Harlan is available to answer your questions about RF and Microwave engineering. If he doesn't have the answer, he will find an industry expert who does.

FROM: Lance Shiells, Worldspace

G'Day Harlan

Hope you can help, I am confused as to how you obtain twice the power rating of a single SSPA when two sspa'a outputs are fed into a combiner, for example in redundancy mode. I am thinking here, the antenna load hasn't changed, and the input RF voltage shouldn't change, so whats happening?

Cheers

Lance

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Dear Lance,

If two sspa's (or sources of any sort) of equal amplitude are fed in-phase into a power combiner, the output will be the sum of the two sources, less resistive and mismatch losses. It is important to note, however, that in a redundancy mode, if one source drops out, the output power will only be half of the remaining source (or 1/4 the original combined power) since the other half will be absorbed by the matching termination of the combiner. The key to combining power is the phase and amplitude coherence of the two inputs.

FROM: Mike Rozbicki, Fluor Corporation

Harlan,

I'm planning a microwave radio link between a fixed offshore platform and a ship that is attached to a mooring tower. This ship weathervanes 360 degrees.

Is there a gyrostabilized parabolic antenna platform available that would keep the dish looking at the platform?

Could I get by with an omnidirectional antenna on the ship?

The distance between both sites would be about 4KM.

Thank you for your assistance.

Mike

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Dear Mike,

I'm sure that someone can make a gyrostabilized parabolic antenna, but it is likely to be very expensive. If the link budget will permit it, you will be better off with an omni-directional antenna on the ship, even at the expense of higher transmitter power. I suggest that you have a look at: Microwave Radio Transmission Design Guide, Trevor Manning, Artech House, 1999, ISBN# 1-58053-031-1.

There are different bands allocated by the FCC for different types of services. The choice of frequency is partially dependent on regulations and partially dependent on the type of link design when there is a regulatory choice. The main difference between MW media and fibre optic links is that fibre optics generally have more information bandwidth, however, they are hard wired rather than free space. I suggest that you have a look at: Microwave Radio Transmission Design Guide, Trevor Manning, Artech House, 1999, ISBN# 1-58053-031-1.

FROM: Bhushan Deshpande, Astra Microwave Products Ltd

Sir,

In most of the mixer applications, the LO power is mentioned as ex:LO power +7dbm

Why do we have to give the mentioned power levels? What will happen if we give lesser power?

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Dear Bhushan,

The LO power level is specified to provide the proper bias level for the diodes. Using a lesser power level will increase mismatches and will also cause saturation to occur at a lower level.

FROM: Mohan Goyal, Rockwell International

How [does one] measure second harmonics intercept point of a amplifier?

I am John Faidros,an undergraduate student at the DUTH, Greece. I am trying to make a noise jammer at 9GHz. For this purpose I have to make a noise source at DC-10MHZ using a step recovery diode. Could you please give me an example because I am in serious trouble with this project?

Thanks a lot.

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Dear John,

I would suggest that you should use a noise diode that is specifically designed to provide white Gaussian noise rather than a standard SRD. I don't normally suggest specific suppliers, however, there aren't many sources for noise diodes. I suggest that you go to www.noisecom.com and click on noise diodes. You will see a data sheet with specs and a typical bias circuit. Good luck with your project.

FROM: Michael Wu, Askey

Dear Harlan;

Can we measure S parameters and noise figure using probe without SMA connector in circuit?

Or any other method we can use conveniently in practical?

Best regards

Michael

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Dear Michael,

Yes! There are commercial probes available from several sources that will permit you to make measurements without a connector. You can check in our on-line buyer's guide. There are also suppliers listed in the CAD, Test & Measurement Supplement to the November 2003 issue of Microwave Journal.

FROM: Majid OstadRahimi, Hamawave

Dear Harlan,

Hope to be good. Thanks for your attention to my question.

I have a question about mm wave antennas:

Which kinds of antennas can be used in mm waves for example 30GHz to 100GHz?

Why "stripline" with TEM mode of propagation is limited up to X band for circuit implementation , while "microstrip" that is quasi TEM mode of propagation can be used up to 110 GHz? (I read this in some handbooks)

Best,

Majid O.R.

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Dear Majid,

You can use many types such as horns, dielectric lenses, patches or printed arrays. However, most people use dishes at those frequencies.

Your handbook information is incorrect. There is no reason why stripline cannot be used at higher frequencies as long as the ground plane spacing is small enough to prevent undesired moding. I have personally built stripline circuits up to 40 GHz with good results.

FROM: Sofia Vatti, University of Patras

Dear Harlan,

I have adressed you this question in the past (around 27th of November) but have not yet received an answer.

I am building a dual-loop frequency synthesizer at 5GHz. At an attempt to gather information on the advantages of such a PLL design, I came across a paper of the IEEE International Symposium on Circuits and Systems. Not having the authority (login and password) to download such a paper, would it be easy for you to acess this particular paper and e-mail it to me? Should you be aware of any other relevant papers, do snd then as well.

I am sending you the title of the paper and the link.

T.Aytur and John Khoury, “Advantages of Dual-loop Frequency Synthesizers for GSM Applications”, IEEE International Symposium on Circuits and Systems, June 9-12, 1997,Vol.1.

http://ieeexplore.ieee.org/xpl/tocresult.jsp?isNumber=13317

Best regards,

Sofia

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Dear Sofia,

In order to download that paper, you either must be an IEEE member or pay a fee to IEEE. It would not be proper for me to use my personal IEEE membership to circumvent their requirements and provide the paper to you. I suggest that you contact IEEE.

FROM: Duncan Simpson, simpco. maint.

Harlan, how is it possible to disrupt the electomagnetics of a rf system in regards to remote viewing? what other components besides a power amp, osilloscope, voice modulator, wave form generator, an antenna, monitor and a volt meter if any that makes up A psychotronic weapon?

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Dear Duncan,

It is quite possible to disrupt a RF system, however, I don't know what a "psychotronic weapon" is. There is quite a bit of research being done on a variety of electromagnetic weapons. The cover story in the November 2003 issue of IEEE SPECTRUM magazine is an excellent overview article on the subject.

FROM: Giovanni Milotta

Hi Mr Harlan,

I'd like to have one example or a suitable article or book about the correct usage of noise power specification of operational amplifiers (nV/sqrrootHz) on phase noise calculation of PLL.

Thanks

Giovanni Milotta

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Dear Giovanni,

Phase noise is a function of several contributors including phase detector noise, loop filter noise and the op-amp noise. They are all expressed in Voltage units/sqrrootHz. There is a software package with an instruction manual that will calculate phase noise based on these inputs. It is: PLL: Linear Phase-locked Loop Control System Analysis, Unruh, Artech House, 1991, ISBN# 0-89006-522-5.

FROM: Fred Jones, Transportation Technologies Inc.

Sir:

Is it possible to use microwave to stimulate a sign made from fiber optic or other material from distances of a mile or more? If not fiber optic material, could you recommend some other material? Is micro wave sight limited?

Thank you for your time and expertise.

Fred F. Jones

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Dear Fred,

While microwaves can be used to stimulate some phosphors and gases, the power levels needed are far too great to be used at any distance. To answer your second question, microwave transmissions are limited to line of sight.

FROM: Graeme Payne, GK Enterprises

Your December 16 Q&A had a question from Aladin Assisi about Scientific-Atlanta and their microwave antenna measurement products business. You mentioned that the company had been replaced by MI Technologies.

While I am sure that is probaly true with respect to those specific products, it is not true of the company itself. Scientific-Atlanta is very much alive and well, in Lawrenceville, Georgia northeast of Atlanta. I have had ocasion to drive past their new campus three times in the past month. Their current product line is in the networking and broadband television content distribution markets.

Graeme C. Payne Lilburn, Georgia, USA

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Dear Graeme,

You are correct. I was refering only to the antenna measurement products.

FROM: Hoover Chen, Araftek, Inc.

Dear Mr. Howe,

How are you? I apologize if I ask you a not so "microwave-centric" questions.

I would like to know the GSM handset manufacturing industries status in South Africa.

Are there any GSM handset manufacturing companies in South Africa? Everyone knows there are many such manufacturers in South Korea, Taiwan and China. But are there any companies in South Africa that designs, assembles the GSM handset phones?

Our company, Araftek, located in Fremont, CA., is a start-up company that designs and manufactures GSM power amplifier module for the GSM handset. I am trying to find who makes GSM handsets in South Africa. I would appreciate it very much if you can refer me to whoever who knows the industry well inside South Africa.

Best Regards,

Hoover Chen Araftek, Inc.

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Dear Hoover,

I am not aware of any South African companies in the GSM handset business, although some of the major international companies may have factories there. I suggest that you contact the South African Chamber of Business at www.sacob.co.za.

Which are the hot topic of research in the microwave research (theoretical research)?

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Dear Khalid,

In my view, the hot topics are nanotechnology, continued compound semiconductor research and MEMS.

FROM: Gaby Rabayov, Tadiran Spectralink Ltd

Do you know a site that has RF high power amp layout recommendations?

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Dear Gaby,

I am not aware of any site that will provide this sort of information. There is a good book on high power amplifier design, which may be helpful to you. RF Power Amplifiers for Wireless Communications, Steve Cripps, Artech House, 1999, ISBN # 0-89006-989-1.

FROM: Gal Teofil Ovidiu, University of Oradea

Can you help me with free microwave circuit design software?

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Dear Gal,

Sonnet Software offers some free software along with their other products. They can be reached at www.sonnetsoftware.com.

I am delighted to hear from you and that things are well. It has been a long time since I've seen you. To re-instate your subscription, please vist our Online Subscriptions service center. Best wishes!